Fuel oil, liquid chemicals and the like are stored in tanks. In recent years, for example, a centralized oiling system for collective housing has been proposed. In this system, kerosene is supplied to respective home from a centralized kerosene tank through pipes.
The tank may suffer some cracks due to time degradation. In this case, liquid in the tank leaks from the tank. It is very important to detect such leakage as soon as possible and cope with it adequately for preventing explosion and fire hazard, ambient pollution, or generation of poisonous gas.
As a detection device for detecting leakage of liquid in a tank in the shortest possible time, JP(A)-2003-185522 (Patent Document 1) has proposed a configuration that includes a measurement pipe into which liquid in a tank is introduced and a measurement slim-pipe provided below the measurement pipe and measures the liquid flow rate inside the measurement slim-pipe using a sensor section additionally provided to the measurement slim-pipe to detect a minute variation of the liquid surface in the tank, i.e., a liquid level variation.
In this liquid leakage detection device, an indirectly heated flowmeter is used as a sensor additionally provided to the measurement slim-pipe. In this flowmeter, a current is applied to heat a heating element, and a part of the heating value is allowed to be absorbed by liquid. Then, the heat absorption value of the liquid varies in accordance with the liquid flow rate. This characteristic is used to detect influence of the heat absorption based on a variation in an electrical characteristic value such as a resistance value caused by a temperature variation of a temperature-sensitive element.
However, in the indirectly heated flowmeter used in the liquid leakage detection device disclosed in the above Patent Document 1, a variation in an electric circuit output level with respect to a variation in a liquid flow rate becomes small in the region where the flow rate value is as infinitesimal as, e.g., 1 milliliter/h or less, so that an error in the flow rate measurement value tends to increase. Thus, there is a limit to an improvement in leakage detection accuracy.
Further, there is a known flow rate sensor that measures fluid flow rate based on temperature difference between two fixed points. In the flow rate measurement based on temperature difference between two fixed points, fluid is heated by a heat source disposed at a given position in a pipe, temperature sensitive elements are disposed at an appropriate interval on the upstream and downstream sides of the heater with respect to the fluid distribution direction in the pipe, and the fluid flow rate is measured based on a temperature difference which is caused between the upstream-side temperature sensitive element and downstream-side temperature sensitive element when the fluid in the pipe is distributed. However, when the flow rate value becomes 3 milliliter/h or more, a variation in an electric circuit output level with respect to a variation in a liquid flow rate becomes small, so that an error in the flow rate measurement value tends to increase in the large flow rate region (that is, the rate of the flow rate difference that can be discriminated when the measurement is carried out is increased to reduce the sensitivity).
Patent Document 1: JP(A)-2003-185522